A high-voltage switch may include a vacuum interrupter module which performs the operation of current interruption. As is well known in the art, electric furnaces require large amounts of power to be under constant control. To properly maintain this control, the switch must be able to withstand repeated interrupting of contact at voltages of up to 138 kV.
An exemplary high-voltage vacuum interrupter module of the prior art is shown in FIG. 1 and generally indicated by the numeral 10. The module 10 includes an upper terminal pad 12 that provides an electrical and mechanical connection to the furnace or other piece of equipment operating at high voltage. The upper terminal pad 12 is electrically connected to a vacuum interrupter, generally indicated by the numeral 14, through a stationary stem 18. The vacuum interrupter 14 includes a vacuum container 20 through which axially extends the stationary stem 18 that is electrically connected to a stationary contact 24. The stationary contact 24 is mateable with a moving contact 26. FIG. 1 shows the contacts 24 and 26 in a closed circuit condition. As those skilled in the art will appreciate, the moving contact 26 connects to and is mated with the stationary contact 24 to complete the current path to the furnace or the like. The moving contact 26 and the stationary contact 24 are sealed within the highly evacuated vacuum container 20. The moving contact 26 is connected to a moving stem 32 that axially extends from the vacuum container 20. Attached to the moving stem 32 is a shunt plate 58. A flexible shunt 34 is connected to the shunt plate 58 and connected at its opposite end to a lower terminal pad 38. The module 10 is part of a switching mechanism that may include any number of modules.
A cylindrical, hollow housing, which is generally indicated by the numeral 40, along with the upper terminal pad 12 and the lower terminal pad 38, enclose the vacuum interrupter 14 and the flexible shunt 34. The terminal pad 38 may provide circular grooves that receive O-rings to preclude entry of moisture into the housing 40.
A bellows 41 is incorporated between the moving stem 32 and the proximal end of the vacuum container 20. The bellows 41 is a very thin flexible metal that allows the contacts 24 and 26 to separate while still maintaining the very high vacuum inside the vacuum container 20. External air pressure acting on the bellows 41 exerts a force on the moving stem 32 which is proportional to the diameter of the bellows. This external air pressure force has to be overcome by the switch mechanism during the opening of the module 10. Thus, sizing of the bellows 41 is critical. A dielectric material 42 is interposed and bonded to the housing 40 and the vacuum interrupter 14 to preclude any electrical flashover caused by system transients.
In operation, when the module 10 changes from a closed to an open state, an external lever mechanism moves the moving stem 32 by an insulated pull rod 44 through a pull rod screw 57. The pull rod 44 axially withdraws the pull rod screw, which is mechanically and electrically attached to the moving contact 26 inside the vacuum interrupter 14, and separates the contacts 24 and 26 from one another a small amount. Movement of the pull rod screw is biased by the flexible shunt 34. At the moment of separation, metal in the contacts 24 and 26 is vaporized and forms a conductive plasma. The current continues flowing through this plasma until a current zero is reached. At the current zero, metal vapors are no longer generated at the surfaces of the contacts 24 and 26. As such, the conductive plasma dissipates, and when the next alternating current cycle occurs, no current flows because there is no conductive material in the gap between the contacts.
Closing of the module 10 initiates a reverse sequence of the above operation. The external lever mechanism pushes on the pull rod 44 which pushes on the pull rod screw which in turn pushes on an over-travel spring 46 contained within the moving stem 32. Axial movement of the moving stem 32 axially moves the moving contact 26. Accordingly, the contacts 24 and 26 are connected and the vacuum interrupter module 10 is closed. The spring 46 functions to equalize the force among several vacuum interrupter modules simultaneously operated by the same external switch mechanism.
The switch usually fails because of wear to the vacuum interrupter 14 or some of its attached electrical components. The high-voltage module 10 is then discarded and replaced with a new module. The average useful life of the prior art module is estimated to be in the range of about 60,000 to 125,000 operation cycles. Replacement of the vacuum interrupter modules is quite expensive, especially considering that only one or two of the internal parts have failed.
Another problem with known high voltage modules relates to the dielectric material 42 disposed between the vacuum interrupter 14 and the housing 40. Since the dielectric material is bonded to both components, it is impossible to simply replace the defective vacuum interrupter without damaging the dielectric material. Accordingly, simple replacement of the vacuum interrupter 14 is not possible.
Another drawback with existing high-voltage switches is that the connection between the upper terminal pad 12 and the vacuum interrupter 14 must be provided with as low a resistance connection as possible. The prior art upper terminal pad 12 is typically made of aluminum that over time might develop a highly resistive aluminum oxide layer. Prior to assembly of the switch, the oxide surface is chemically stripped and an oxide inhibiting grease is applied thereto. The prior art vacuum interrupter module 10 has a copper mounting plate 50 that is mechanically fastened to the aluminum terminal pad with bolts 52. Thus, maintaining a low electrical resistance joint with this design depends on the mechanical connection between the terminal pad and the mounting plate and the effectiveness of the oxide inhibiting grease. Over time, the effectiveness of either feature could be lost, thereby increasing the joint's electrical resistance. This leads to failure of the vacuum interrupter module 10.
Yet another drawback of the original vacuum interrupter module 10 is that a single copper moving stem 32 makes both the electrical and mechanical connections between the pull rod screw to the rest of the switch mechanism. In the vacuum interrupter module 10, a steel pin 56 is inserted through a transverse hole in the copper moving stem and through a slotted hole in the pull rod screw 57. The copper moving stem 32 is comparatively softer than the steel pin 56 as a result of annealing during the brazing operations used to fabricate the vacuum interrupter. With the numerous opening and closing operations of the vacuum interrupter, a large amount of force is exerted on the steel pin. These repeated cycles on the pin enlarge the hole in which it is retained, thus changing the dimensions of the connecting pieces. When multiple vacuum interrupter modules are controlled by a single lever mechanism, the moving stem holes enlarge at a different rate. As a result, there is a loss of synchronism between the contacts in each vacuum interrupter module 10 as their respective stem holes deform. Prior art vacuum interrupter modules attempt to counteract this enlarging of the stem hole by locating it tangentially to the attached shunt plate 58. This provides an increased bearing surface on one side of the pin. Alternatively, the module would be cycled through a number of operations prior to synchronizing the switch to cold work and increase the strength of the material surrounding the steel pin 56.
Several drawbacks are evident from this modified shunt plate construction. Welding the shunt plate to the moving stem and maintaining a precise tangential alignment is difficult because the exact location of the shunt plate after welding is controlled by weld shrinkage. Moreover, trying to maintain a consistent weld shrinkage all around the stem so that the shunt plate is simultaneously tangential to the holes on both sides of the stem is extremely difficult and cannot be uniformly achieved.
Thus, the need exists for a refurbished vacuum interrupter module with improved operating characteristics. Additionally, there is a need for a refurbished vacuum interrupter module which can recycle useable parts at a significant savings.